Reactions of Alkanes: Unlike alkanes, alkenes are very reactive and they undergo addition reactions to form a saturated compound. While alkanes, alkenes and cycloalkanes undergo combustion reactions with oxygen, only alkenes can participate in addition reactions. Alkynes: Addition Reactions The principal reaction of the alkynes is addition across the triple bond to form alkanes. A hydrogen atom joins to one of the carbon atoms originally in the double bond, and a halogen atom to the other. These addition reactions are analogous to those of the alkenes. All alkenes undergo addition reactions with the hydrogen halides. Heterolytic bond cleavage occurs and one of the halogens obtains positive charge and … Addition of \(HBr\) to an alkene is not stereoselective: the reaction results in racemization at both of the alkene carbons. Depending on the structure of the starting alkene, electrophilic addition has the potential to create two new chiral centers. Let's take a look at a few examples so you can see what an addition reaction looks like. In an addition reaction, two compounds come together to form one larger compound. This is called hydrogenation Hence alkenes undergo electrophilic addition reactions. See image A. As halogen molecule, for example Br 2, approaches a double bond of the alkene, electrons in the double bond repel electrons in bromine molecule causing polarization of the halogen bond. Alkene + hydrogen → alkane. Alkenes can react with different types of chemicals during addition reactions. Consider the addition of \(HBr\) to cis-3,4-dimethyl-3-hexene. Let's look at the reaction of hydrogen bromine (HBr) and propene (C_3H_6). A typical addition reaction may be illustrated by the hydrochlorination of propene (an alkene), for which the equation is CH 3 CH = CH 2 + HCl → CH 3 C + HCH 3 + Cl − → CH 3 CHClCH 3 . Alkanes (the most basic of all organic compounds) undergo very few reactions. The pi bond is under strain and tries to convert into two sigma bonds. The two reactions of more importaces is combustion and halogenation, (i.e., substitution of a single hydrogen on the alkane for a single halogen) to form a haloalkane. Here, HBr will become the electrophile. Here, we will discuss the addition reactions of alkenes. For example, with ethene and hydrogen chloride, you get chloroethane: Figure 7.7.1 Electrophilic addition of HCl to ethene. Since the hydrogen in HBr is s… Addition of Halogens: Halogenation of Alkenes. Hence they attract electrophile easily. This creates a dipolar moment in the halogen molecule bond. Bromine hogs the electrons in the HBr bond, thus making the hydrogen slightly positive and the bromine slightly negative. Addition reactions to alkenes and alkynes are sometimes called saturation reactions because the reaction causes the carbon atoms to become saturated with the maximum number of attached groups.
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